Atlas Image mosaic obtained as part of the Two Micron All Sky Survey
(2MASS), a joint project of the University of Massachusetts and the
Infrared Processing and Analysis Center/California Institute of
Technology, funded by the National Aeronautics and Space Administration
and the National Science Foundation.

System Summary

R Coronae Australis lies in the
north central edge (19:01:53.65-36:57:07.62, ICRS 2000.0) of
Constellation
Corona
Australis (see
David
Malin's photo, also at
Astronomy
Picture of the Day), the Southern Crown -- northwest of
Rukbat
(Alpha Sagittarii) and southeast of
Kaus
Australis (Epsilon Sagittarii). The star is located in the Corona
Australis molecular complex, one of the closest star-forming regions,
which is conspicuously isolated from the crowded galactic plane. A
highly elongated system of dark molecular clouds with six condensations,
the complex has a centrally-condensed core ("condensation A") centered
near R Coronae Australis. This core measures more than one by two
light-years (ly) (0.4 by 0.7 parsecs) in extent and contains about
50 Solar masses of molecular gas
(Wilking
et al, 1997; and
Andreazza
and Vilas-Boas, 1996).

The star lies with a nearby
star-forming region, surrounded
by a bluish reflection nebula
and embedded in a large dust
cloud
(more).

Space-based, HIPPARCOS parallax measurements that have a high standard
error (Plx=122, +/- e_Plx=68 mas) suggest that R Coronae Australis lies
around 30 ly from Sol. Astronomers studying the star, which is
obscured by interstellar dust, place more confidence on Earth-based
parallax measurements and other analysis indicating that the star's
location may be around 500 ly away. Such a farther distance estimate
is in better agreement with the estimated parallaxes of other stars
in the Coronae Australis complex (which may include the isolated nearby
neutron and/or quark star
RX J1856.5-3754), whose mean
distance has been estimated to be around 420 ly -- 130 parsecs
(Marraco
and Rydgren, 1981).

As catalogued and defined in
Thé
et al (1994), R Coronae Australis is defined as a "Herbig Ae/Be"
pre-main sequence star with two to 10 times the mass of Sol, which
George Howard Herbig
originally classified as embryonic stars ("located in obscured
regions") illuminating fairly bright nebulosity in their immediate
vicinity and of larger mass than the T-Tauri stars already well-known
at the time
(Herbig,
1960). Of spectral and luminosity type B8 -- previously A5-F0 --
IIpe
(Grady
et al, 1996, in
pdf,
page 176), this star is still contracting towards the main sequence
(see H-R
diagram for pre-main sequence tracks for seven stars of different
mass). Although R Coronae Australis is 40 times brighter than Sol
(Ward-Thompson
et al, 1985), its high luminosity is obscured because the star is
accreting matter within an enormous surrounding shell of gas and dust,
much of which has already been expelled into space from the poles after
being drawn into the forming star via a vast circumstellar disk
(Grady
et al, 1996, in
pdf; and
Mitskevich,
1995). There is a bi-polar outflow of molecular gas
associated with plasma jets
(Hartigan
and Graham, 1987, page 4; Walker et al, 1985; and
Ward-Thompson
et al, 1985).
The star is an irregular variable with more frequent outbursts during
times of greater average brightness, but it also has a long-term
periodic variation of about 1,500 days and about 1/2 magnitude that may
be linked to changes in its circumstellar shell, rather than to
stellar pulsations
(Bellingham
and Rossano, 1980). R Coronae Australis has at least twice Sol's
mass.

It is likely that any protoplanetary bodies that may have formed around
the R Coronae Australis star are still agglomerating other
planetesimals. Hence, any developing carbon-based life on a developing
Earth-type planet would be subject to tremendous heat and intense
asteroidal and cometary bombardment.
Useful star catalogue numbers for the star include: R CrA, Hip 93449,
CD-37 13027, CP(D)-37 8452, Hen3-1733, GCRV 11457,
and IRAS 18585-3701.

A star is probably born as a nebular cloud of gas and dust of
interstellar size collapses under gravitational forces (often after a
disturbance such as shockwaves from a nearby supernova explosion),
and spins inward via an accretion disk towards an increasingly dense
core. While still obscured from view by dust, nuclear fusion may
ignite at the center of these pre-stellar objects, as hydrogen is fused
into helium. Proto-stars (pre-main sequence T-Tauri and more massive
stars -- e.g., Herbig Ae/Be stars such as R Coronae Australis ) are
born as the energy of core hydrogen fusion pushes outward to balance
the inward pull of gravity.

This newborn star is about half a million years old.
The star itself is obscured by its protoplanetary disk,
which is viewed edge-on at the bottom of the image as
a flattened cloud of dust split into two halves by a
dark lane. All that is visible is the reflection of
the star's light by dust above and below the plane of
the disk, which has a diameter of about 450 AUs.

Before a star finishes forming, around a tenth of the matter around it
may be ejected by infalling through its accretion disk and then being
blown out by bi-polar jets
(more NASA
photos) to produce two giant lobes of molecular gas, and bow
shocks from the jets hitting the surrounding stellar nebula
(painting).
Known as
Herbig-Haro
objects since their discovery in the early 1950s, these lobes typically
extend a few light-years in length, have masses similar or larger than
the developing star itself, and are moving apart at speeds of tens to a
few hundred kilometers (several to tens of miles) per second.
Stretching for several light-years, the bi-polar jets may be driven at
supersonic speeds by an intense magnetic field at the axis of rotation
of an embryonic star less than a few hundred thousand years old.

The matter moving outward may carry angular momentum away from the
developing star and allow accretion to continue, as well as churn up
the surrounding nebula and so provide the necessary turbulence to slow
down its collapse. Eventually, the supply of infalling matter will run
out and shut down the star's bi-polar jets. Subsequently, much of the
surrounding gas and dust that remain around a new star will be blown
away, by the young star's radiation in T-Tauri winds after core nuclear
fusion is turned on.

At around this time, however, large dust grains may be beginning to
agglomerate together in a process that leads to planetesimals,
proto-planets, and the final planetary system. Hence, once fusion
begins at a star's core, planets may have only a few hundred thousand
years or so to form from proto-planetary bodies before the dusty
circumstellar disk becomes too tenuous, and the star enters the main
sequence. Young stars in the solar neighborhood include: many OBA
type stars such as Beta Pictoris
(A3-5 V), which is young enough to have an easily detectable dust disk;
Epsilon Eridani (a K2 V with a dust ring as
wide as 60 times the Earth to Sun distance that is estimated to be
between one half and one billion years old); and the double-binary
system of HD 98800 which has four main-sequence K and M stars that may
be only 10 million years old.

Despite the position of Corona Australis in the Southern Hemisphere,
the Ancient Greeks had named this constellation as the crown of the
neighboring centaur, Constellation Centaurus.
For more information about the stars and objects in this constellation
and an illustration, go to Christine Kronberg's
Corona
Australis. For another illustration, see David Haworth's
Corona
Australis.

For more information about stars including spectral and luminosity
class codes, go to ChView's webpage on
The Stars of
the Milky Way.